Method of fabricating a modulator of the propagation losses and of the index of propagation of an optical signal

ABSTRACT

This method of fabricating a modulator comprises:
         following the bonding of a substrate onto an encapsulated semiconductor layer containing a first electrode of the modulator and prior to the formation of a second electrode of the modulator, the method comprises the removal ( 522 ) of a base substrate onto which the encapsulated semiconductor layer is deposited in such a manner as to expose a face of a buried layer of dielectric material, situated under the buried semiconductor layer, without modifying the thickness of the buried layer by more than 5 nm, and   the formation ( 524, 528 ) of the second electrode is implemented directly on this exposed face of the buried layer such that, once the second electrode has been formed, it is the buried layer which directly forms a dielectric layer interposed between proximal ends of the electrodes of the modulator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/319,902, filed Jan. 23, 2019, which is a continuation ofInternational Application No. PCT/FR2017/052100, filed Jul. 27, 2017,which is based upon and claims the benefit of priority from FrenchPatent Application 1657626, filed Aug. 8, 2016, the entire contents ofeach of which are incorporated herein by reference.

The invention relates to a method of fabricating a modulator of thepropagation losses and of the index of propagation of a guided opticalsignal and also to a modulator fabricated by this method.

“Propagation loss” denotes the optical losses experienced by the opticalmode propagating in a waveguide within which it is guided.

‘Index of propagation’ denotes the effective index of propagation of theoptical mode propagating in a waveguide within which it is guided.

Such known modulators comprise a waveguide formed by the stackingimmediately on top of one another of:

-   -   a proximal end of a first electrode,    -   a thin dielectric layer, and    -   a proximal end of a second electrode.

By applying a potential difference between the first and secondelectrodes, the density of charge carriers at the interfaces between thedielectric layer and the proximal ends of the first and secondelectrodes is modified. This leads to a modification of the propagationlosses and of the index of propagation experienced by the guided opticalfield propagating in the waveguide. Typically, the dielectric layer is alayer of silicon dioxide.

The known methods of fabrication of such a modulator comprise:

-   -   providing a stack successively comprising a base substrate, a        buried layer of dielectric material and a semiconductor layer,        the thickness of the buried layer being equal to e_(2ini) to        within approximately 5 nm, where e_(2ini) is a constant, then    -   etching the semiconductor layer so as to structure a first        electrode of the modulator within this semiconductor layer, this        first electrode having a proximal end, a distal end and an        intermediate part which extends, in a transverse direction, from        the proximal end up to the distal end so as to mechanically and        electrically connect these ends, then    -   encapsulating the structured semiconductor layer in a dielectric        material so as to obtain a semiconductor layer encapsulated        within a dielectric material in which layer the dielectric        material extends, in the transverse direction, until it directly        touches the proximal end of the first electrode, then    -   bonding a substrate onto the encapsulated semiconductor layer,        then    -   forming a second electrode of the modulator having a proximal        end facing the proximal end of the first electrode, these        proximal ends being separated from each other only by a        dielectric layer in such a manner as to form a waveguide able to        guide the optical signal to be modulated.

For example, such a method of fabrication is disclosed in theapplication WO2011037686.

The method of the application WO 2011037686 is advantageous because itallows modulators to be fabricated in which the dielectric layer isthin, in other words less than 25 nm thick. This is advantageous becausesuch a modulator then exhibits both a good modulation efficiency and lowpropagation losses.

On the other hand, the implementation of the known methods offabrication described in these applications leads to modulators whoseperformance characteristics are dispersed. The term “performancecharacteristics” here denotes notably the bandwidth of the modulator andthe modulation efficiency of this modulator. The modulation efficiencyis the ratio between the variation in index of refraction induced pervolt applied between the two electrodes of the modulator.

“Dispersion of the performance characteristics” denotes the fact thatthe performance characteristics vary from one modulator to another anddo so even if all these modulators are fabricated by the same method offabrication. Such variations are typically caused by inaccuracies duringthe fabrication process which modify, in a random and uncontrolledmanner, the capacitance of the capacitor formed by the dielectric layerinterposed between the two electrodes of the modulator, and the positionof the maximum intensity of the optical field guided by the modulator.For example, the inaccuracy could be in the thickness of the dielectriclayer or in the positions of the electrodes with respect to each other.

Prior art is also known from:

-   -   US 2015/0055910A1,    -   WO2005/091057A1,    -   WO2015/194002A1,    -   WO2013/155378A1,    -   U.S. Pat. No. 8,363,986B2, and    -   FR2867898.

The invention aims to provide a method of fabrication which reduces thedispersion in the performance characteristics of the modulatorsfabricated by this method. In other words, the objective is to obtain amethod of fabrication which renders the performance characteristics ofthe modulators fabricated more repeatable. One of its subjects istherefore such a method in accordance with claim 1.

In the method claimed, the dielectric layer is directly formed by theburied layer constructed prior to the formation of the first electrode.The phrase “directly” formed by a buried layer denotes the fact that thedielectric layer is obtained without substantially modifying the initialthickness e_(2ini) of the buried layer. The expression “withoutsubstantially modifying the initial thickness” denotes the fact that theinitial thickness e_(2ini) of the buried layer, which forms thedielectric layer interposed between the electrodes of the modulator, isneither increased nor decreased by more than err_(p) with respect to itsinitial thickness e_(2ini), where err_(p) is a constant equal to 5 nm or4 nm and, preferably, equal to 3 nm or 1 nm. In particular, in themethod claimed, the dielectric layer is not the result of the formationon the first electrode of a layer of dielectric material, nor is it theresult of the thinning of a thicker dielectric layer. For this reason,the thickness of the dielectric layer is controlled with an increasedprecision with respect to the known methods of fabrication ofmodulators. In addition, the thickness of the dielectric layer is moreuniform. This results in the dispersion of the capacitance of thecapacitor of the modulator being much lower when the method claimed isimplemented than with the known methods of fabrication. This accordinglyallows modulators whose performance characteristics are more repeatableto be fabricated. In particular, the bandwidth and the modulationefficiency of the modulators fabricated by the method claimed are muchless dispersed than if they had been fabricated by a known method suchas that described in the application WO 2011037686 or US 20150055910.

Moreover, the method claimed allows the first electrode of the modulatorto be encapsulated in a dielectric material without having to leave anempty space next to this first electrode. This is particularlyadvantageous since, in the known methods such as that of the applicationUS 20150055910 where such an empty space exists, this empty space islocated under the second electrode. However, an empty space between thesecond electrode and the base substrate creates a stray capacitancewhich degrades the performance characteristics of the modulator.

The embodiments of this method of fabrication may comprise one or moreof the features of the dependent claims.

The embodiments of the method of fabrication claimed may furthermoreoffer one or more of the following advantages:

-   -   The method claimed allows a layer of thermal silicon oxide to be        used as a dielectric layer, which improves the bonding of the        second electrode and hence, ultimately, the performance        characteristics of the modulator.    -   The fact of having a proximal end thicker than the intermediate        part of the same electrode renders the method less sensitive to        the errors of positioning of one electrode with respect to the        other electrode. More precisely, it allows a finer control of        the position of the maximum intensity of the optical field        guided by the modulator and hence the efficiency of the        modulator fabricated. Thanks to this feature, the performance        characteristics of the modulators fabricated are even more        repeatable.    -   The fact of having a more highly doped proximal end only in a        region close to the dielectric layer allows, for the same        performance characteristics, the propagation losses of the        modulator fabricated to be reduced with equal access        resistances, or the access resistance to be decreased with equal        optical propagation losses, while at the same time keeping the        distribution of the optical field unchanged.    -   The fact that the thickness of the dielectric layer is less than        25 nm or 15 nm allows modulators with a good modulation        efficiency to be obtained.    -   The fact that the modulator and a laser source comprising a        waveguide made of III-V material coupled to an optical waveguide        via the same dielectric layer are fabricated at the same time        furthermore allows the characteristics of this laser source to        be made repeatable without adding any additional step to the        method of fabrication of the modulator.

Another subject of the invention is a modulator in accordance with claim9.

The invention will be better understood upon reading the descriptionthat follows, given solely by way of non-limiting example and presentedwith reference to the drawings, in which:

FIG. 1 is a schematic illustration of a transmitter as a vertical crosssection;

FIG. 2 is a schematic illustration, seen from above, of a modulator andof a phase-matching device of the transmitter in FIG. 1;

FIG. 3 is an enlarged illustration, as a vertical cross section, of themodulator of the transmitter in FIG. 1;

FIG. 4 is a flow diagram of a method of fabrication of the transmitterin FIG. 1;

FIGS. 5 to 16 are schematic illustrations, as vertical cross sections,of various states of fabrication obtained during the implementation ofthe method in FIG. 4;

FIGS. 17 to 21 are schematic illustrations, as vertical cross sections,of other possible embodiments of the modulator in FIG. 3;

FIG. 22 is a schematic illustration, as a vertical cross section, ofanother embodiment of the transmitter in FIG. 1.

In these figures, the same references are used to identify the sameelements. In the remainder of this description, the features andfunctions well known to those skilled in the art are not described indetail.

In this text, the expressions “the layer is made of material M”, “layerof material M” or “M layer” denote a layer in which the material Mrepresents at least 90%, and preferably at least 95% or 99%, of the massof this layer.

FIG. 1 shows a transmitter 5 for an optical signal modulated in phaseand/or in amplitude in order to transmit bits of information to areceiver by means of an optical fiber for example. For this purpose, thetransmitter 5 comprises a laser source 7 which emits an optical signalwhose phase and/or amplitude is subsequently modulated by a system 6 forphase and/or amplitude modulation of this optical signal.

For example, the wavelength λ_(Li) of the optical signal emitted by thelaser source 7 is in the range between 1250 nm and 1590 nm.

Typically, the laser source 7 is a DBR (distributed Bragg reflector)laser or DFB (distributed feedback) laser. Such a laser source is wellknown and only the details necessary for understanding the invention aredescribed here. For example, for general details and the operation ofsuch a laser source, the reader can refer to the following articles:

-   Xiankai Sun and Amnon Yariv: “Engineering supermode silicon/III-V    hybrid waveguides for laser oscillation”, Vol. 25, No. 6/June    2008/Journal of the Optical Society of America B.-   B. Ben Bakir et al., “Hybrid Si/III-V lasers with adiabatic    coupling”, 2011.-   B. Ben Bakir, C. Sciancalepore, A. Descos, H. Duprez, D. Bordel, L.    Sanchez, C. Jany, K. Hassan, P. Brianceau, V. Carron, and S. Menezo,    “Heterogeneously Integrated III-V on Silicon Lasers”, Invited Talk    ECS 2014.

In order to simplify FIG. 1 and the following figures, only a hybridlaser waveguide 200, 220 and a surface grating coupler 8 of the lasersource 7 are shown.

Such a coupler 8 is for example described in the following article: F.Van Laere, G. Roelkens, J. Schrauwen, D. Taillaert, P. Dumon, W.Bogaerts, D. Van Thourhout and R. Baets, “Compact grating couplersbetween optical fibers and Silicon-on-Insulator photonic wire waveguideswith 69% coupling efficiency”. It is formed in an encapsulatedsemiconductor layer 3. By design, the coupler can emit upward ordownward. In this description, it is inverted, but it may be chosen bydesign to make it emit upward or downward. The layer 3 here comprisesstructured single-crystal silicon encapsulated in a dielectric material116. Generally speaking, a dielectric material has an electricalconductivity at 20° C. of less than 10⁻⁷ S/m and, preferably, less than10⁻⁹ S/m or 10⁻¹⁵ S/m. In addition, in the case of the dielectricmaterial 116, its index of refraction is strictly less than the index ofrefraction of silicon. For example, in this embodiment, the dielectricmaterial 116 is silicon dioxide (SiO₂).

The layer 3 extends horizontally and directly over a rigid substrate 44.In the layer 3, the single-crystal silicon is situated in one and thesame horizontal plane parallel to the plane of the substrate 44. Here,the single-crystal silicon of the layer 3 is also mechanically andelectrically isolated from the substrate 44 by a thickness of thedielectric material 116. Typically, the maximum thickness ofsingle-crystal silicon in the layer 3 is in the range between 100 nm and800 nm. In this example, the maximum thickness of the single-crystalsilicon in the layer 3 is equal to 500 nm.

In FIG. 1 and the following figures, the horizontal is represented bydirections X and Y of an orthogonal reference frame. The direction Z ofthis orthogonal reference frame represents the vertical direction. Inthe following, the terms such as “upper”, “lower”, “above”, “under”,“high” and “low” are defined with respect to this direction Z. The terms“left” and “right” are defined with respect to the direction X. Theterms “front” and “back” are defined with respect to the direction Y.

FIG. 1 shows the elements of the transmitter 5 in cross section in avertical plane parallel to the directions X and Z.

The substrate 44 extends horizontally. It is formed of a successivestacking of a base substrate 441 and of a layer 442 of dielectricmaterial. The thickness of the base substrate 441 is typically greaterthan 80 μm or 400 μm. For example, the base substrate 441 is a siliconbase substrate. The layer 442 is made of silicon dioxide. The thicknessof the layer 442 is typically greater than 500 nm or 1 μm or even more.

The hybrid laser waveguide 200, 220 is composed of a waveguide 200formed from a III-V gain material and from a waveguide 220 made ofsingle-crystal silicon. Generally speaking, the waveguide 200 is usedfor generating and amplifying an optical signal inside of an opticalcavity of the laser source 7. Here, for this purpose, it is formed in alayer 36 comprising a III-V gain material encapsulated in a dielectricmaterial 117. For example, the material 117 is silicon dioxide orsilicon nitride. This layer 36 extends horizontally directly over adielectric layer 20. The layer 20 itself extends horizontally directlyover an upper face of the layer 3.

The thickness of the layer 20 is typically in the range between 5 nm and25 nm and, preferably, between 10 nm and 25 nm. Here, the thickness ofthe layer 20 is equal to 20 nm.

The layer 36 typically comprises a doped lower sub-layer 30, a stack 34of quantum wells or quantum dots made of a quaternary alloy and an uppersub-layer 35 doped with a dopant of opposite sign to that of thesub-layer 30. The sub-layers 30 and 35 here are formed of doped InP.

In FIG. 1, only a strip 33, a stack 233 and a strip 234 formed,respectively, in the sub-layer 30, the stack 34 and the sub-layer 35 areshown. This superposition of the strip 33, the stack 233 and the strip234 constitutes the waveguide 200.

The waveguide 200 also comprises:

-   -   bump contacts 243G and 243D in direct mechanical and electrical        contact with the strip 33 and situated, respectively, to the        left and to the right of the stack 233, and    -   a bump contact 244 in direct mechanical and electrical contact        with the strip 234.

These contacts 243G, 243D and 244 allow an electrical current to beinjected into the stack 233 between the contacts 243G, 243D and thecontact 244.

The waveguide 220 is formed in the single-crystal silicon of the layer3. This waveguide 220 extends under the strip 33. Typically, itsthickness and its width vary along Y as described in the articlepreviously cited by Ben Bakir et al. In FIG. 1, the waveguide 220 isshown, by way of illustration, in the particular case where thedirection of propagation of the optical signal inside of this waveguideis parallel to the direction Y. For example, for this purpose, thewaveguide 220 adopts a configuration known by the term “rib”. Thus, thetransverse cross section of this waveguide, parallel to the plane XZ,has a central spine 222 from which thinner lateral arms 223G and 223Dextend on either side, parallel to the direction X. Here, the waveguide220 is separated from the strip 33 only by a portion of the layer 20.For example, the waveguide 220 is optically connected to the waveguide200 by an adiabatic or evanescent coupling. For a detailed descriptionof an adiabatic coupling the reader can refer to the article previouslycited by X. Sun and A. Yariv or to the following article: Amnon Yariv etal., “Supermode Si/III-V hybrid lasers, optical amplifiers andmodulators: proposal and analysis” Optics Express 9147, vol. 14, No. 15,Jul. 23, 2007. The characteristics of the optical coupling between thewaveguide 220 and the waveguide 200 depend notably on the dimensions ofthe waveguide 220 and, in particular, on the thickness of the centralspine 222. It is therefore important that the thickness of this spine222 can be adjusted independently of the dimensions of the otherphotonic components formed on the same substrate 44. For example, here,the thickness of the spine 222 is equal to the maximum thickness of thesingle-crystal silicon in the layer 3, in other words here to 500 nm.

The system 6 may be a system for modulation of the phase only, or of theamplitude only or simultaneously of the phase and of the amplitude. Inorder to modulate the phase or the amplitude of the optical signal, thesystem 6 comprises at least one modulator of the propagation losses andof the index of propagation of a guided optical signal and, often, atleast one phase-matching device. For example, the system 6 is aMach-Zehnder interferometer in which the modulator 100 and thephase-matching device 300 are arranged in one of the branches of thisinterferometer for modulating the amplitude and/or the phase of theoptical signal generated by the laser source 7. The structure of aMach-Zehnder interferometer is well known and is not described here.Accordingly, in order to simplify FIG. 1, only one modulator 100 and onephase-matching device 300 are shown.

The device 300 allows the phase of an optical signal propagatingparallel to the direction Y inside of a waveguide 320 to be adjusted.For example, the waveguide 320 is longer in the direction Y than it iswide in the direction X. The waveguide 320 is formed from thesingle-crystal silicon of the layer 3. Here, its thickness is forexample equal to the thickness of the bulging part 222. The index ofrefraction of silicon varies strongly as a function of temperature.Thus, by varying the temperature of the waveguide 320, the index ofpropagation of the optical signal in this waveguide may be modified andhence the phase of the optical signal adjusted. For this purpose, thedevice 300 comprises two heaters 322G and 322D each disposed on onerespective side of the waveguide 320. Here, the heater 322D can bederived from the heater 322G by symmetry with respect to a verticalplane parallel to the directions Y and Z and going through the middle ofthe waveguide 320. Thus, only the heater 322G will now be described inmore detail with reference to FIGS. 1 and 2.

The heater 322G comprises an arm 324 which extends, parallel to thedirection X, from a proximal end 56 up to a distal end 58. The arm 324also extends parallel to the direction Y. The arm 324 is formed from thesingle-crystal silicon of the layer 3.

The proximal end 56 is in direct mechanical contact with the waveguide320. Here, the proximal end 56 touches one vertical side of thewaveguide 320. For this purpose, the arm 324 and the waveguide 320 forma single block of material.

The thickness of the proximal end 56 is less than the maximum thicknessof the waveguide 320 in such a manner as to confine the optical signalwithin the waveguide 320. For example, the thickness of the proximal end56 is 1.5 times or two times or three times or four times smaller thanthe maximum thickness of the waveguide 320.

The distal end 58 is doped in order to render the single-crystal siliconresistive and to form an electrical resistance which forms a singleblock of material with the waveguide 320. In FIG. 1, the doped regionsof the single-crystal silicon are finely hatched and appear dark. Theshortest distance between this doped region of the arm 324 and thewaveguide 320 is, for example, strictly greater than 200 nm or 400 nm.

In order to make an electrical current flow inside the distal end 58,the heater 322G also comprises two bump contacts 51G and 52G in directmechanical and electrical contact with the distal end 58. Here, thesecontacts 51G and 52G are situated one behind the other in the directionY and at each end of the distal end 58 in this direction Y. The bumpcontacts of the heater 322D, shown in FIG. 2, respectively carry thereferences 51D and 52D.

When a current, conducted via the contacts 51G and 52G, passes throughthe distal end 58, the latter transforms a part of the electrical energythus received into heat which propagates by thermal conduction throughthe proximal end 56 as far as the waveguide 320. Thus, the heater 322Gallows the waveguide 320 to be heated without any resistive elementbeing implanted in the waveguide 320 or in immediate proximity to thiswaveguide. The device 300 allows the phase of the optical signal in thewaveguide 320 to be adjusted slowly. On the other hand, it does notallow a fast variation of the phase of the optical signal.

Conversely, the modulator 100 allows a fast modification of the phase ofthe optical signal. For this purpose, it comprises two electrodes 120and 130. These electrodes 120 and 130 can also be seen, as a top view,in FIG. 2.

The electrode 120 is formed from the single-crystal silicon of the layer3. It extends, in the direction X, from a proximal end 12 up to a distalend 11 going via a thinned intermediate part 13. It also extends in thedirection Y. In the direction Y, its transverse cross section remainsconstant. Parallel to the plane XZ, the transverse cross sections of theends 11 and 12 and of the intermediate part 13 are each rectangular. Theends 11, 12 and the intermediate part 13 are flush with the plane upperface of the layer 3. The ends 11, 12 and the intermediate part 13 aretherefore in direct contact with the lower face of the layer 20.

The intermediate part 13 connects the ends 11 and 12 together, bothmechanically and electrically. Its thickness e₁₃ is chosen in such amanner as to laterally confine (cross section X-Z) the intensitydistribution of the optical field within the proximal end 12. For thispurpose, the thickness e₁₃ is less than 0.8e₁₂ and preferably less than0.5e₁₂ or than 0.25e₁₂, where e₁₂ is the maximum thickness of the end12. The thickness e₁₃ is also typically greater than 70 nm in order todecrease the electrical resistance between the ends 11 and 12. Thisresistance is referred to as the “access resistance”. For this purpose,the thickness e₁₃ is often greater than 0.1e₁₂ or 0.15e₁₂. Here, thethickness e₁₁ of the distal end 11 is equal to the thickness e₁₂. Inthis embodiment, the thickness e₁₂ is equal to 300 nm and the thicknesse₁₃ is equal to 150 nm or 100 nm. The horizontal lower face of theintermediate part 13 is separated from the substrate 44 only by thedielectric material 116.

Here, the distal end 11 is more highly doped than the proximal end 12.For example, the concentration of dopant in the end 11 is in the rangebetween 10¹⁷ and 2×10¹⁹ atoms/cm³. The concentration of dopant in theend 12 is for example in the range between 10¹⁷ and 2×10¹⁸ atoms/cm³.

The electrode 130 is made of a doped semiconductor material with adoping of opposite sign to that of the electrode 120. Here, it is formedfrom InP in the sub-layer 30. The dopant concentration of the electrode130 is, for example, in the range between 10¹⁷ and 2×10¹⁸ atoms/cm³ orbetween 10¹⁷ and 5×10¹⁸ atoms/cm³.

The electrode 130 extends, parallel to the direction X, from a proximalend 32 up to a distal end 31. The electrode 130 also extends in thedirection Y. It is directly situated on the layer 20. Parallel to theplane XZ, its transverse cross section is rectangular. In the directionY, this transverse cross section is constant.

The proximal end 32 is situated facing the proximal end 12 and extendsbeyond this end 12, in the direction X, in such a manner as to exhibit aprotrusion 32 d (FIG. 3) facing the intermediate part 13. Typically, theprotrusion 32 d is at least 5 nm or 10 nm or 25 nm long in the directionX. The proximal end 32 is separated from the proximal end 12 and fromthe intermediate part 13 only by a portion of the layer 20 interposedbetween these proximal ends.

With respect to a vertical plane parallel to the directions Y and Z andgoing through the ends 12 and 32, the distal end 31 is situated on oneside of this plane, whereas the distal end 11 is situated on the otherside. The ends 11 and 31 are not, therefore, facing each other.

In the embodiment in FIG. 1 and the following embodiments, the region34, which extends vertically from the end 31 down to the substrate 44,only comprises solid dielectric materials. Here, these are dielectricmaterials 116 and the layer 20. In FIG. 1, the region 34 has beenhighlighted by filling it with circles. However, there is nodiscontinuity between the dielectric materials situated inside of theregion 34 and those situated outside of this region 34.

The superposition, in the direction Z, of the end 12, of a portion ofthe layer 20 and of the end 32 is dimensioned in order to form awaveguide 70 capable of guiding the optical signal generated by thelaser source 7 in the direction Y. The waveguides 70 and 320 are forexample optically connected together via an adiabatic coupler not shownhere.

The maximum thickness of the proximal ends 12 and 32 is chosen such thatthe point M, where the maximum intensity of the optical field of theoptical signal propagating in the waveguide 70 is located, is as closeas possible to the layer 20. Preferably, the point M is situated at thecenter of the portion of this layer 20 interposed between the ends 12and 32. Indeed, it is at the interfaces between the ends 12, 32 and thelayer 20 that the density of charge carriers is maximal when a potentialdifference is present between these proximal ends. Thus, by placing thepoint M at this location, the efficiency of the modulator 100 isimproved. The maximum thickness e₃₂ of the proximal end 32 is generallyin the range between 50 nm and 300 nm. In this embodiment, the indicesof refraction of the ends 12 and 32 are close to one another.Accordingly, the maximum thicknesses of the ends 12 and 32 are chosen tobe substantially equal so that the point M is situated inside of thelayer 20. For example, the maximum thickness e₁₂ of the proximal end 12is in the range between 0.5e₃₂ and 1.5e ₃₂, and, preferably, between0.7e₃₂ and 1.3e ₃₂. Here, the thicknesses e₁₂ and e₃₂ are chosen to bothbe equal to 300 nm.

The intermediate part 13 allows a better control of the position of thepoint M in the direction X and hence limits the dispersion in theperformance characteristics of the modulators 100 during theirfabrication. More precisely, the position of the point M in thedirection X is essentially fixed by the width W₁₂ (FIG. 3) of theproximal end 12 in the direction X. Indeed, the limited thickness e₁₃ ofthe intermediate part 13 confines the optical signal inside the end 12.The width W₁₂ is, by etching, defined to within +/−δ, where δ is anerror equal, typically, to +/−5 nm, or +/−10 nm. Conversely, if theintermediate part 13 has the same thickness as the end 12, the width W₁₂is defined by the width of the coverage of the electrodes 130 and 120.However, the positioning of the electrode 130 is determined, for itspart, by lithographic alignment with a precision δal typically equal to+/−25 nm, or +/−50 nm. Hence, in the absence of a thinned intermediatepart, the error on the width W₁₂ is +/−μal and the dispersion in theperformance characteristics of the modulators 100 is greater. Thisconfiguration of the electrode 120 therefore allows a lower sensitivityto the errors in positioning of the electrode 130. In particular, themodulation efficiency depends directly on the width W₁₂. As aconsequence, the modulation efficiency is less dispersed thanks to theintermediate part 13 and to the protrusion 32 d.

Furthermore, this embodiment also allows a better control of thecapacitance of the modulator.

The modulator 100 also comprises two bump contacts 21 and 22, in directmechanical and electrical contact with the distal ends 11 and 31,respectively. These contacts 21 and 22 are connected to a source ofvoltage controllable as a function of the bit or bits of information tobe transmitted by the transmitter 5.

One possible operation of the transmitter 5 is as follows. The lasersource 7 generates an optical signal. At least a part of this opticalsignal is directed toward a Mach-Zehnder interferometer at least one ofthe branches of which comprises, successively, the modulator 100 and thephase-matching device 300. This part of the optical signal is thereforesuccessively guided by the waveguide 70, then the waveguide 320, beforebeing recombined with another part of the optical signal guided by theother branch of the Mach-Zehnder interferometer so as to form themodulated optical signal. For example, the waveguides 70 and 320 areoptically coupled together via an adiabatic coupler. At the output ofthe Mach-Zehnder interferometer, the optical field may be coupled to anoptical fiber via a waveguide similar to the waveguide 320, then by thesurface grating coupler 8.

A method of fabrication of the transmitter 100 will now be describedwith reference to FIGS. 4 to 16. FIGS. 5 to 16 show various states offabrication of the transmitter 5 as a vertical cross section parallel tothe directions X and Z.

During a step 500, the method begins by providing a substrate 4 (FIG.5). Here, this substrate 4 is an SOI (silicon-on-insulator) substrate.The substrate 4 comprises, directly stacked one on top of the other inthe direction Z:

-   -   a base substrate 1 of silicon, conventionally with a thickness        greater than 400 μm or 700 μm,    -   a buried layer 2 of thermal silicon dioxide of thickness        e_(2ini), and    -   a layer 43 of single-crystal silicon which, at this stage, has        not yet been etched or encapsulated in a dielectric material.

The thermal silicon dioxide is an oxide of silicon obtained by oxidationof the base substrate 1 at a high temperature, in other words higherthan 650° C. or 800° C. By virtue of the nature of this oxide, the layer2 exhibits two noteworthy properties:

1) its thickness, even when it is thin, remains uniform, and2) it allows a direct bonding of better quality to be obtained.

The “uniform” thickness of the layer 2 means that, at any point on thelayer 2, its thickness is in the range between e_(2ini)−err_(2ini) nmand e_(2ini)+err_(2ini) nm, where:

-   -   e_(2ini) is a constant, typically equal to the average thickness        of the layer 2, and    -   err_(2ini) is a constant less than or equal to 5 nm and,        preferably, equal to 3 nm or 1 nm.

Direct bonding is a method of bonding in which two wafers are bondeddirectly onto each other without adding an intermediate layer ofadhesive. The bonding results from the appearance of chemical bondsdirectly between the two faces of these wafers. Generally, after havingbeen brought into mechanical contact with one another, the wafersundergo a thermal treatment in order to reinforce the bonding.

Generally, the thickness e_(21n1) is greater than 7 nm or 10 nm and,typically, less than 100 nm or 50 nm. Here, the initial thicknesse_(21n1) of the layer 2 is equal to 20 nm+/−1 nm and the thickness ofthe layer 43 is equal to 500 nm. Such a thickness e_(2ini) and such aprecision on the thickness of the layer 2 is obtained by theconventional methods of fabrication of SOI substrates.

During a step 502, a localized doping of the layer 43 is carried out.Here, a first localized doping operation 504 is initially carried out,during which doped regions 506 (FIG. 6) with the same doping are formedin the layer 43. These regions 506 are only formed at the locations ofthe future arms of the matching device 300 and of the electrode 120 ofthe modulator 100. These regions 506 have a doping level equal to thatof the distal end 58 and of the proximal end 12.

Subsequently, a second operation 508 for doping of the layer 43 iscarried out in such a manner as to obtain a region 510 (FIG. 7) morehighly doped than the regions 506. The region 510 here is partiallysuperposed onto one of the regions 506. For example, the region 510 isobtained by applying a new implantation on a part of one of the regions506. The region 510 is formed at the location of the future distal end11 of the electrode 120. The doping of the region 510 here is equal tothe doping of the distal end 11.

During a step 514, the layer 43 undergoes a first localized partialetching (FIG. 8) so as to thin the thickness of the silicon at thelocations of the electrode 120 and of the arms 324 of the heaters 322Gand 322D. At the end of the step 514, the regions 506 and 510 arethinned and have a thickness less than the initial thickness of thelayer 43. Here, the thickness of the thinned regions 506 and 510 isequal to the thickness of the electrode 120 and of the arms 324, inother words 300 nm.

During this first localized partial etching, the thickness of the layer43 is also thinned in non-doped regions, for example in order to formthe patterns of the future surface grating coupler 8 and the lateralarms 223G and 223D of the waveguide 220. On the other hand, during thisstep 514, other regions referred to as “non-thinned” are not etched andconserve their initial thickness. In particular, these non-thinnedregions are situated at the location of the spine 222 of the waveguide220 and at the location of the waveguide 320.

Still during this step 514, the layer 43 subsequently undergoes a secondlocalized partial etching in order to thin the thickness of the silicononly at the location of the future intermediate part 13. At the end ofthis second localized partial etching, the thickness of the layer 43 atthe location of the future intermediate part 13 is 150 nm.

During a step 516, a localized total etching of the layer 43 is carriedout (FIG. 9). In contrast to the partial etching, the total etchingcompletely eliminates the thickness of silicon of the layer 43 in theunmasked regions where it is applied. Conversely, masked regions protectthe layer 43 from this total etching. This total etching is carried outin such a manner as to structure, simultaneously in the layer 43, thewaveguides 220 and 320, the arms of the matching device 300, the surfacegrating coupler 8 and the electrode 120. For this purpose, only theregions corresponding to these various elements are masked. At the endof this step, the state shown in FIG. 9 is obtained.

During a step 518, the layer 43 of single-crystal silicon, which hasbeen structured during the preceding steps, is encapsulated in silicondioxide 116 (FIG. 10). The layer 3 is then obtained comprisingstructured single-crystal silicon encapsulated in the dielectricmaterial 116. The upper face of the material 116 is subsequentlyprepared for bonding, for example for direct or molecular bonding. Forexample, the upper face of the material 116 is polished by means of amethod such as a CMP (Chemical-Mechanical Polishing) method.

During a step 520, the upper face of the substrate 4, in other words atthis stage the polished face of the material 116, is subsequently bondedonto the outer face of the substrate 44 (FIG. 11), for example by director molecular bonding. The substrate 44 has already been described withreference to FIG. 1.

During a step 522 (FIG. 12), the base substrate 1 is removed in order toexpose a face of the layer 2 and this is done without substantiallymodifying the initial thickness e_(2ini) of the layer 2. For thispurpose, the base substrate 1 is eliminated via two successiveoperations: a first operation for coarse removal of the majority of thethickness of the base substrate 1 so as to leave only a residual thinlayer of the base substrate 1 remaining on the layer 2. Typically, afterthe first operation, the thickness of this residual layer is less than40 μm or 30 μm. This first operation is referred to as “coarse” sincethe precision on the thickness of the residual thin layer is rough, inother words greater than ±0.5 μm or ±1 μm but, generally, still lessthan ±4 μm and, preferably, less than ±3 μm. Given that the precisionrequired on the thickness of the residual thin layer is rough, a quickor low-cost method of removal may be used. Typically, the operation forcoarse removal is an operation for thinning of the base substrate 1 bymechanical polishing. Here, at the end of this first operation, theresidual thin layer has a thickness of 20 μm to within ±2 μm.

Subsequently, a second finishing operation is implemented in order tocompletely eliminate the residual thin layer of the base substrate 1without modifying the thickness of the layer 2. Typically, the secondoperation is a very selective chemical etching operation. Thedescription “very selective” denotes the fact that the chemical agentused during this operation etches the base substrate 1 at least 500times, and preferably at least 1000 or 2000 times, faster than the layer2. Here, the chemical agent used is TMAH (Tetramethylammoniumhydroxide). TMAH etches silicon 2000 times faster than thermal siliconoxide. In this embodiment, in order to be sure of removing the entiretyof the residual thin layer, the very selective etching is adapted so asto etch 22 μm of silicon, i.e. 2 μm more than the theoretical thicknessof the residual thin layer. This choice leads, in the worst casescenario, to an over-etching of 2 nm (=2 μm/2000) of the thickness ofthe layer 2. Indeed, since the thickness of the residual thin layer is20 μm±2 μm, this means that in some places the thickness may be 18 μm.If it is planned to etch 22 μm, then the over-etching at the locationwhere the thickness is 18 μm may reach 4 μm. Hence, the over-etching oflayer 2 at this point is of 2 nm. Thus, after the step 522, the layer 2is exposed and forms the layer 20. Its thickness is equal to 20 nm±3 nm,in other words±1 nm due to the imprecision on the initial thicknesse_(2ini) of the layer 2 to which the imprecision of ±2 nm due to theover-etching is added. With respect to other methods of obtaining a thinlayer, this method has the advantage of providing a thin layer whosethickness is much more uniform. Indeed, when the thin layer is obtainedby thinning of a thicker layer of oxide or by growth of a thin layer ofoxide on a face of an encapsulated structured layer (see for exampleWO2011037686 or US2015055910), the thickness is much less uniform.Typically, with the known methods, the thickness of the layer of oxideis controlled, at best, to within ±10 nm or ±20 nm.

For this reason, the dispersion in the performance characteristics ofthe modulators fabricated according to this method is much smaller thanthat obtained with the known methods.

Lastly, advantageously, outgassing cavities are sunk into the layer 20outside of the locations where the electrode 130 and the strip 33 are tobe formed. Typically, these cavities traverse the layer 20 verticallyfrom one side to the other. Their role is to trap the gaseous elementsgenerated during the direct bonding of a layer onto the layer 20. Thus,these cavities allow a bonding of better quality to be obtained on thelayer 20. In order to simplify the figures, these cavities have not beenshown in these figures.

At the end of the step 522, a stack of the substrate 44 and of thelayers 3 and 20 (FIG. 12) is obtained.

During a step 524, a layer 36A (FIG. 13) of III-V gain material isformed on the layer 20. For example, the layer 36A is bonded onto thelayer 20 on top of the waveguide 220 and of the electrode 120. The layer36A comprises the sub-layer 30 of doped InP with a doping of oppositesign to that of the electrode 120, the stack 34 and the sub-layer 35.

During a step 526, a localized etching (FIG. 14) of the sub-layer 35 andof the stack 34 is carried out in order to structure the strip 234 inthe sub-layer 35 and the stack 233 in the stack 34. During this step,the sub-layer 30 is not etched.

During a step 528, a localized total etching (FIG. 15) of the sub-layer30 is carried out in order to simultaneously structure the strip 33 andthe electrode 130 in this sub-layer. The precision δal of thepositioning of the electrode 130 with respect to the electrode 120depends on the tools and techniques used to perform this step. Thisprecision δal is therefore known in advance. The length of theprotrusion 32 d of the electrode 130 depends on this precision δal.Here, the desired position of the electrode 130 is chosen in such amanner that the target length of this protrusion 32 d is greater than orequal to the absolute value of the precision δal. Thus, irrespective ofthe error in positioning which occurs during the fabrication of themodulator 100, the protrusion 32 d is systematically created as long asthe alignment error remains within the predictable range±δal.

During the step 528, a part or the entirety of the thickness of thelayer 20 situated between the electrode 130 and the strip 33 may beremoved. However, this has no consequence on the thickness of theportions of the layer 20 interposed between the electrodes 120 and 130and between the waveguide 220 and the strip 33.

During a step 530, the structured layer 36A is encapsulated (FIG. 16) inthe dielectric material 117. The layer 36 comprising the III-V gainmaterial encapsulated in the dielectric material 117 is then obtained.

Lastly, during a step 532, the bump contacts 21, 22, 51G, 52G, 51D, 52D,243G and 243D are formed. The transmitter 5 such as is shown in FIG. 1is then obtained.

This method of fabrication offers numerous advantages. In particular:

-   -   It allows the thickness of the layer 20 to be precisely        controlled and a particularly plane layer 20 to be obtained        because said layer is formed on the side of the layer 3 which        has the same level everywhere, which simplifies the bonding of        the layer 36A.    -   It allows the thickness of the electrode 120 to be adjusted        independently of the thickness of the waveguide 220 and, more        generally, independently of the thickness of the layer 43 of        single-crystal silicon. This is particularly useful since,        generally speaking, in order to improve the operation of the        laser source 7, the waveguide 220 must be thick enough, in other        words here of the order of 500 nm, and the strip 33 must be thin        enough, in other words here of the order of 300 nm or 150 nm.        Conversely, in order to improve the operation of the modulator        100, as explained hereinabove, the thickness of the electrode        120 and, in particular, of its proximal end 12, must be chosen        as a function of the thickness of the proximal end 32. Here, the        thickness of the proximal end 32 is imposed by the thickness of        the sub-layer 30 of crystalline InP. It is therefore 300 nm or        150 nm.    -   This method does not increase the complexity of the fabrication        of the transmitter 5. For example, it allows the strip 33 of the        waveguide 200 and the electrode 130 of the modulator 100 to be        formed in one and the same etching operation. Similarly, the        electrode 120 and the waveguide 220 are fabricated        simultaneously during the same etching operation.

FIG. 17 shows a modulator 550 able to replace the modulator 100. Themodulator 550 is identical to the modulator 100 except that theelectrode 120 is replaced by an electrode 552. The electrode 552 isidentical to the electrode 120 except that the distal end 11 is replacedby a distal end 554 whose thickness is equal to the thickness e₁₃ of theintermediate part 13. Thus, the distal end 554 and the intermediate part13 are a continuation of each other and form only a single block ofrectangular transverse cross section.

FIG. 18 shows a modulator 560 able to replace the modulator 100. Themodulator 560 is identical to the modulator 550 except that theelectrode 552 is replaced by an electrode 562. The electrode 562 isidentical to the electrode 552 except that the proximal end 12 isreplaced by a proximal end 564. The proximal end 564 is identical to theproximal end 12 except that it comprises a more highly doped region 566and a more lightly doped region 568 stacked on top of one another in thedirection Z. The region 566 is directly in contact with the layer 20.The region 568 is situated on the side opposite to the layer 20. Thedoping of the region 566 is the same as that described for the proximalend 12 so as to conserve the same modulation efficiency. The doping ofthe region 566 or 568 denotes the mean density per unit volume ofdopants in this region. In order to limit the access resistance, whileat the same time limiting the optical losses inside the waveguide, thethickness e₅₆₆ of the region 566 is preferably equal to the thicknesse₁₃ of the intermediate part 13 to within ±10% or ±5%. Its thickness isalso generally greater than 70 nm. Here, its thickness is equal to thethickness e₁₃.

The doping of the region 568 is at least two times lower, and preferably4 or 10 times lower, than the doping of the region 566. Typically, theregion 568 is not doped or very lightly doped.

The thickness of the region 568 is equal to e₁₂-e₅₆₆. This configurationof the doping of the proximal end 564 allows the propagation losses inthe modulator 560 to be reduced without substantially modifying itsother performance characteristics such as the modulation efficiency andthe modulation speed, nor does this modify the access resistance at theproximal end 564. Such a configuration of the doping of the proximal end564 is, for example, carried out during the step 502, in other words byimplementing a doping at the location of the proximal electrode 564 inthe layer 43 of silicon such that only the region 566 is doped. Forexample, the regions 566 and 568 are obtained by varying the energy ofimplantation of the dopant and the dose of dopant implanted so as toadjust both the density of dopants and the depth at which the maximumdensity of dopants is situated.

FIG. 19 shows a modulator 570 able to replace the modulator 100. Themodulator 570 is identical to the modulator 100 except that theelectrode 120 is replaced by an electrode 572. The electrode 572 isidentical to the electrode 120 except that the intermediate part 13 isreplaced by an intermediate part 574. The thickness of the intermediatepart 574 is equal to the thickness e₁₂. In this embodiment, therepeatability of the performance characteristics of the modulator 570 istherefore obtained only by virtue of the better control of the thicknessof the layer 20.

FIG. 20 shows a modulator 580 able to replace the modulator 100. Thismodulator 580 is identical to the modulator 100 except that theelectrode 130 is replaced by an electrode 582. The electrode 582 isidentical to the electrode 130 except that a thinned intermediate part584 is introduced between the proximal end 32 and distal end 31. Theintermediate part 584 is, for example, structurally identical to theintermediate part 13.

In addition, in this embodiment, the proximal end 32 is more lightlydoped than the distal end 31. Such a different doping between the ends31 and 32 may be obtained by carrying out a step for localized doping onthe end 31 just after the step 528 and prior to the step 530.

In this embodiment, as in the previous embodiments, the distal end 32comprises the protrusion 32 d which is situated above the intermediatepart 13 of the electrode 120. Under these conditions, the position ofthe maximum intensity of the optical field in the direction X is stillcontrolled by the width W₁₂ of the proximal end 12.

FIG. 21 shows a modulator 590 able to replace the modulator 100. Themodulator 590 is identical to the modulator 580 except that theelectrode 120 is replaced by an electrode 592. The electrode 592 isidentical to the electrode 572 of the modulator 570. The electrode 592has a protrusion 12 d which extends, in the direction X, underneath theintermediate part 584. This protrusion 12 d is configured like theprotrusion 32 d. Thus, in this embodiment, the position of the maximumintensity of the optical field guided by the modulator is controlled bythe width W₃₂ of the proximal electrode 32 rather than by the width ofthe proximal end 12. This offers the same advantages in terms ofrepeatability of the performance characteristics of the modulatorsfabricated as what has already been explained in the case of theembodiment in FIG. 1 and of the proximal end 12.

The method of fabrication of the modulator 590 is for example identicalto that in FIG. 4 except that:

-   -   during the step 514, the second localized partial etching        intended to thin the intermediate part 13 is omitted, and    -   during the step 528, a second localized partial etching able to        thin the intermediate part 584 is implemented in addition to the        localized total etching.

FIG. 22 shows a transmitter 600 identical to the transmitter 5 exceptthat an encapsulated semiconductor layer 602 is interposed between thelayer 20 and the layer 36. The layer 602 comprises a structuredsemiconductor layer 604 encapsulated in silicon oxide. The layer 604 isdirectly in contact with the layer 20. Here, the layer 604 is a layer ofpolycrystalline silicon (polysilicon). This layer is structured, forexample by localized total etches such as those previously described, inorder to form an electrode 608 of the modulator and a spine 610 of thelaser source. The electrode 608 is for example identical to theelectrode 130 except that it is made from polysilicon. The electrode 608is doped by implantation after etching. The spine 610 is not doped.

The spine 610 is situated on top of the waveguide 220 and opticallycoupled to this waveguide 220 through the layer 20 so as to form abi-material waveguide 612. Here, the bi-material waveguide 612 is formedfrom single-crystal silicon and from polysilicon.

The waveguide 200 made of III-V material is directly deposited or bondedonto the layer 602 on top of the bi-material waveguide 612 and opticallycoupled to this waveguide 612. In this embodiment, the electrode 608 isnot formed from the same material as that of the strip 33 of thewaveguide 200.

Variants of the Modulator:

The modulator 100 may be a ring modulator. For this purpose, thewaveguide 70 is closed on itself so as to form an annular waveguide inwhich the density of the charge carriers may be modified as a functionof the potential difference applied between the contacts 21 and 22.Typically, this annular waveguide is connected to a waveguide in whichthe optical signal to be modulated propagates via an evanescentcoupling. In this case, the phase-matching device 300 may be omitted.The waveguide 70 may also form only a limited portion of the annularwaveguide.

In another embodiment, the modulator is used to modulate the intensityof the optical signal passing through it. This is because a modificationof the density of the charge carriers within the waveguide 70 alsomodifies the intensity of the optical signal passing through it.

As a variant, the thickness of the end 11 is equal to the thickness ofthe layer 43 of single-crystal silicon. Indeed, in order to center thepoint M, where the maximum intensity of the optical field of the opticalsignal is located, at the center of the layer 20, it is the thickness ofthe ends 12 and 32 that is important. The thickness of the distal ends11 and 31 does not have any particular bearing on this point.

The thickness of the layer 20 may be greater than 25 nm or 40 nm.

In a similar manner to what has been described for the proximal end 564,the proximal end 32 of the electrode 130 may be replaced by an end withone region more highly doped than another. In another variant, only theend 32 comprises two regions with different levels of doping and thedoping of the end 12 is uniform.

Other embodiments of the regions 566 and 568 with different levels ofdoping are possible. For example, in one variant, the doping of theproximal end 564 decreases progressively with increasing distance fromthe layer 20; a doping gradient is thus created. There is not then anyabrupt modification of the density per unit volume of dopants when goingfrom the region 566 to the region 568. On the other hand, the meandensity per unit volume of dopants in the region 566 remains much higherthan the mean density per unit volume of dopants in the region 568.

The doped region of the electrode 120 may extend beyond the proximal end32 in the direction X or not as far as said end.

As a variant, the width W₃₂ of the proximal end 32 of the modulator 580is smaller than the width W₁₂ of the proximal end 12. In this case, theposition of the maximum intensity of the optical field inside of thewaveguide 70 is controlled by the width W₃₂ rather than by the widthW₁₂.

In another variant, the protrusion 32 d or 12 d is omitted. Indeed, thereproducibility of the positioning of the point M is improved even inthe absence of this protrusion 32 d or 12 d.

Other semiconductor materials may be used to form the electrode 120 or130. For example, the two electrodes are formed from InP or frompolycrystalline or single-crystal silicon.

Other dielectric materials may be used for the material 116 and thelayer 20. For example, these could be silicon nitride, aluminum nitride,an electrically-insulating polymer, or Al₂O₃. Moreover, in the case ofthe layer 20, its index of refraction is not necessarily lower than thatof silicon.

In another embodiment, the electrode 130 is made of a semiconductormaterial different from that used to form the strip 33. In this case,the electrode 130 and the strip 33 are not structured in the samesub-layer of III-V material.

Irrespective of the embodiment, it is possible to interchange the N- andP-doped regions.

Variants of the Laser Source:

Other III-V gain materials may be used to form the layer 36. Forexample, the layer 36 is composed of the following stack going frombottom to top:

-   -   a lower sub-layer of N-doped GaAs,    -   sub-layers with quantum dots of AlGaAs, or AlGaAs quantum wells,        and    -   an upper sub-layer of P-doped GaAs.

The III-V material used to form the sub-layer 30 may be different. Forexample, it could be N- or P-doped AsGa. It will also be noted thatP-doped InP exhibits more optical loss than N-doped InP, and that it istherefore preferable to use N-doped InP in the modulator for theelectrode 130.

The waveguide 220 may take a configuration referred to as “strip-mode”,in other words where the lateral arms 223G and 223D are omitted, or anyother configuration capable of guiding an optical signal.

In another variant, the layer 20 is totally eliminated at the placeswhere it is not indispensable for the operation of the transmitter. Forexample, it is totally eliminated except between the proximal ends 12and 32.

Variants of the Method of Fabrication:

The removal of the base substrate 1 may be carried out differently. Forexample, as a variant, the base substrate 1 is etched away by onlyimplementing the finishing operation without implementing the operationof coarse removal. In another variant, the coarse removal is carried outby means of an operation for coarse etching different from thatimplemented during the finishing operation.

The outgassing cavities sunk into the layer 20 may be omitted, notablyif the layer 20 is thicker. Indeed, if the layer 20 is thicker then theuse of outgassing cavities is unnecessary.

As a variant, the electrode 130 and the strip 33 are not formed at thesame time in the same sub-layer 30. For example, during the step 528,only the strip 33 is structured. Subsequently, a semiconductor layer isdeposited or bonded onto the layer 20 at the location of the futureelectrode 130. Subsequently, it is etched in order to obtain theelectrode 130. In this case, the electrode 130 may be made of a materialdifferent from that used for the strip 33 such as crystalline silicon.

The order of the partial and total etching steps may be reversed. Forexample, a first mask is disposed on the layer 43 in order to bound theperiphery of the electrode 120. Then, a localized total etching isperformed in order to construct the vertical sides of this electrode120. Subsequently, a localized partial etching is implemented in orderto thin the intermediate part 13 of the electrode 120. During thislocalized partial etching, a second mask covering at least the proximalend of the electrode 120 is deposited. This second mask leaves theintermediate part 13 exposed.

In another variant, the second localized total etching is replaced by auniform etching of the whole surface of the layer 3 so as to transformthe non-thinned regions into thinned regions and completely eliminatethe thinned regions.

The order of the doping and etching steps may be reversed.

The modulator and the laser source may be fabricated independently ofeach other. For example, the methods of fabrication described here maybe easily adapted for fabricating either only a modulator or only alaser source.

Other Variants:

The layer 442 may be made of other materials than silicon oxide. Forexample, in one advantageous variant, the layer 442 is formed fromaluminum nitride (AlN) which improves the dissipation of the heatgenerated by the laser source 200 toward the substrate 441.

As a variant, a part or the entirety of the bump contacts are formedthrough the substrate 44 rather than through the material 117. In thiscase, with respect to what has been shown in the preceding figures, oneor more electrical bump contacts come out under the substrate.

As a variant, the waveguide 70, 220 or 320 is curved. In this case, theconfiguration of the various elements optically coupled to thesewaveguides is adapted to the radius of curvature of these waveguides.

As a variant, the phase-matching device is omitted or formeddifferently.

The fact that a proximal end thicker than the intermediate part rendersthe method of fabrication more robust with respect to the errors inpositioning of the electrodes may also be exploited for improving othermethods of fabrication of modulators. In particular, this may beimplemented in methods other than those where the dielectric layer isdirectly formed by the buried layer. In particular, the formation of aproximal end thicker than the intermediate part may also be implementedin known methods such as that described in the applications WO2011037686 or US2015/0055910. In the latter case, the intermediate partis thinned during the structuring of the electrode and before it isencapsulated in the silicon oxide.

Similarly, the higher level of doping of the region 566 of the proximalend directly in contact with the dielectric layer 20 may also beimplemented independently of the fact that the dielectric layer isdirectly formed by the buried layer. For example, as a variant, thelayer 20 is firstly removed during the step 522, then a new dielectriclayer is deposited in order to replace the layer 20 removed. During thestep 522, the layer 20 may also be thinned by more than 10 nm.

1. A modulator of propagation losses and of index of propagation of an optical signal, the modulator comprising: a substrate extending in a plane; a semiconductor layer encapsulated in a first dielectric material, the encapsulated semiconductor layer comprising a lower face directly facing the substrate and an upper face facing away from the substrate, the encapsulated semiconductor layer further comprising at least a first electrode of the modulator formed in the semiconductor layer, the first electrode extending, in a transverse direction parallel to the plane of the substrate, from a proximal end of the first electrode to a distal end of the first electrode via an intermediate part of the first electrode and the first dielectric material continuing, in the transverse direction, until the first dielectric material directly touches the proximal end of the first electrode; a second electrode made of semiconductor material having a doping of opposite sign to a doping of the first electrode, the second electrode extending from a proximal end of the second electrode to a distal end of the second electrode via an intermediate part of the second electrode, the proximal end of the second electrode being situated facing the proximal end of the first electrode and the distal end of the second electrode being situated on an opposite side to the distal end of the first electrode with respect to a plane perpendicular to the transverse direction and going through the proximal ends of the first electrode and the second electrode; a buried layer of a second dielectric material forming a second dielectric layer interposed between the proximal ends of the first electrode and the second electrode, a stack including the proximal ends of the first electrode and the second electrode and including the second dielectric layer forming a waveguide configured to guide the optical signal to be modulated; bump contacts in direct electrical contact with, respectively, the distal ends of the first electrode and the second electrode for electrically connecting the first electrode and the second electrode to different electrical potentials to modify a density of charge carriers in the waveguide; wherein for one electrode selected from the group consisting of the first electrode and the second electrode, the proximal end of the selected electrode is thicker than the intermediate part of the selected electrode.
 2. The modulator as claimed in claim 1, wherein the proximal end of the first electrode or the proximal end of the second electrode comprises a first region directly in contact with the second dielectric layer, the first region is more highly doped than a second region of the proximal end of the first electrode or the proximal end of the second electrode further away from the second dielectric layer.
 3. The modulator as claimed in claim 2, wherein thickness of the first region is greater than or equal to 70 nm.
 4. The modulator as claimed in claim 1, wherein the buried layer of the second dielectric material is a layer of thermal oxide obtained by oxidation of a base substrate at over 700° C.
 5. The modulator as claimed in claim 1, wherein, at every point of the buried layer, thickness of the buried layer is equal to e_(2ini) to within 10 nm, wherein e_(2ini) is a constant equal to an average thickness of the buried layer.
 6. The modulator as claimed in claim 1, wherein the proximal end of an electrode non-selected from the group consisting of the first electrode and the second electrode, extends, over a distance of at least 5 nm, in the transverse direction on either side of the proximal end of the selected electrode.
 7. The modulator as claimed in claim 1, wherein the selected electrode is the second electrode.
 8. The modulator as claimed in claim 1, wherein the selected electrode is the first electrode.
 9. The modulator as claimed in claim 1, wherein the proximal end, the distal end, and the intermediate part of the first electrode are flush with the upper face of the encapsulated semiconductor layer.
 10. A modulator of propagation losses and of index of propagation of an optical signal, the modulator comprising: a substrate extending in a plane; a semiconductor layer encapsulated in a first dielectric material, the encapsulated semiconductor layer comprising a lower face directly facing the substrate and an upper face facing away from the substrate, the encapsulated semiconductor layer further comprising at least a first electrode of the modulator formed in the semiconductor layer, the first electrode extending, in a transverse direction parallel to the plane of the substrate, from a proximal end of the first electrode to a distal end of the first electrode via an intermediate part of the first electrode and the first dielectric material continuing, in the transverse direction, until the first dielectric material directly touches the proximal end of the first electrode; a second electrode made of semiconductor material having a doping of opposite sign to a doping of the first electrode, the second electrode extending from a proximal end of the second electrode to a distal end of the second electrode via an intermediate part of the second electrode, the proximal end of the second electrode being situated facing the proximal end of the first electrode and the distal end of the second electrode being situated on an opposite side to the distal end of the first electrode with respect to a plane perpendicular to the transverse direction and going through the proximal ends of the first electrode and the second electrode; a buried layer of a second dielectric material forming a second dielectric layer interposed between the proximal ends of the first electrode and the second electrode, a stack including the proximal ends of the first electrode and the second electrode and including the second dielectric layer forming a waveguide configured to guide the optical signal to be modulated; bump contacts in direct electrical contact with, respectively, the distal ends of the first electrode and the second electrode for electrically connecting the first electrode and the second electrode to different electrical potentials to modify a density of charge carriers in the waveguide; wherein the proximal end of the first electrode or the proximal end of the second electrode comprises a first region directly in contact with the second dielectric layer, the first region is more highly doped than a second region of the proximal end of the first electrode or the proximal end of the second electrode further away from the second dielectric layer. 